Flowmeters are commonly used devices for measuring a flow of a liquid through a pipe. In various branches of industry, for example in the chemical industry or in the pharmaceutical industry, the measurement data obtained by flowmeters is used to control complex industrial processes.
There is a wide range of flowmeters using different measurement principles on the marked, for example differential pressure flowmeters, magnetic inductive flowmeters, sonic- or ultrasonic flowmeters, coriolis or vortex flowmeters.
Electromagnetic flowmeters make use of Faraday's law of induction, which states that a voltage is induced in a conductor moving in a magnetic field. In electromagnetic flowmeters, the flowing medium corresponds to the moving conductor. The induced voltage is proportional to the flow velocity and is detected by two measuring electrodes and transmitted to the amplifier. Flow volume is computed based on the pipe's diameter. The constant magnetic field is generated by a switched direct current of alternating polarity.
Coriolis flowmeters operate on the Coriolis Effect. A mass flow dependent Coriolis force occurs when a moving mass is subjected to an oscillation perpendicular to the flow direction. The measuring system accurately determines and evaluates the resulting effects on the measuring tubes.
Vortex flowmeters operate according to Karman's vortex street principle. Vortices are created and alternate behind a bluff body. The number of vortices shed per time unit, the vortex frequency, is directly proportional to the flow rate.
Ultrasonic flowmeters use acoustic waves or vibrations of a frequency of more than 20 kHz. Depending on the design, they use either wetted or non-wetted transducers on the pipe perimeter to couple ultrasonic energy with the fluid in the pipe.
Some ultrasonic flowmeters operate on the Doppler Effect, whereby the transmitted frequency is altered linearly by being reflected from particles and bubbles in the fluid. The frequency shift is linearly proportional to the rate of flow of materials in the pipe and is used to develop a signal proportional to the flow rate.
In addition, there are flowmeters on the market, that measure the difference in transit time between ultrasonic signals, for example short ultrasonic pulses or beams, transmitted in a single path along and against the flow. Two transducers are used, one upstream of the other. Both act as transmitter and receiver for the ultrasonic signals. The flow velocity is directly proportional to the difference of the upstream and downstream transit times. A product of a cross-sectional area of the pipe and the flow velocity provides a measure of the volumetric flow.
Differential pressure flowmeters comprise a differential pressure sensor and a flow-restricting element inserted in the flow channel, for example an orifice or a nozzle. Typically, the flow-restricting element is a pipe segment of standard length, which is inserted between a first and a second pipe. The diameters of the first and the second type also comply with an industry standard for differential pressure flow measurement. In operation, a fluid to be measured flows through the first pipe, the flow-restricting element and the second pipe. Standardized forms of connections, for example flanges, are foreseen, for providing a fluid tight connection between the flow-restricting element and the first and the second pipe. According to Bemoulli's law, a moving fluid causes a pressure differential across the restriction in the flow channel. This pressure differential is proportional to a square of the volumetric flow rate.
In order to achieve good measurement accuracy it is necessary that the same flow profile occurs at the measurement point at practically all times. This requires sufficiently long straight pipe runs before and behind the measurement point in order to stabilize the flow profile.
Narrowing the flow channel by the restricting element results in a considerable nonrecoverable pressure loss in the fluid.
Despite the disadvantages involved with differential pressure flowmeters there is a reluctancy to replace differential pressure flowmeters by ones that are more suitable. The standard dimensions defined in Industry for differential flowmeters allow for replacement of one differential pressure flowmeter by another. Other types of flowmeters generally do not comply with the standard dimensions for differential pressure flowmeters. Consequently, replacement of a differential pressure flowmeter by another type of flowmeter requires extensive and costly re-designs at the measurement sites.